[TypeChecker] NFC: Bring back SolverShrinkUnsolvedThreshold

Author: xedin

include/swift/Basic/LangOptions.h

@@ -906,6 +906,10 @@ namespace swift {
     /// is for testing purposes.
     std::vector<std::string> DebugForbidTypecheckPrefixes;
 
+    /// The upper bound to number of sub-expressions unsolved
+    /// before termination of the shrink phrase of the constraint solver.
+    unsigned SolverShrinkUnsolvedThreshold = 10;
+    
     /// Enable experimental operator designated types feature.
     bool EnableOperatorDesignatedTypes = false;